The present invention relates to heat sinks, and particularly relates to convection heat sinks, such as may be used for semiconductor cooling.
Heat sinks are widely used to enhance the removal of heat energy from various types of electronic components. For example, heat sinks are pervasively used to cool semiconductor devices such as microprocessors, memory circuits, lasers, charge-coupled devices, and power electronics such as insulated gate bipolar transistors (IGBTs), etc.
Selection of an appropriate heat sink is determined by the specific thermal load to be dissipated. The chosen heat sink must withstand the maximum heat load applied by the component to be cooled while not exceeding the maximum temperature specified by the component supplier/manufacturer. Various classes of heat sinks are available depending on the thermal load requirements. For example, convection heat sinks (both natural and forced air), liquid cooled heat sinks and heat pipe systems are all available. Convection heat sinks are widely used in applications requiring 26 W/in.2 or less of heat dissipating capability (i.e., heat flux). Liquid cooled heat sinks are widely used in applications requiring 40 to 480 W/in.2 of heat dissipating capability. Heat pipes are widely used in applications requiring up to 600 W/in.2 of heat dissipating capability.
Convection heat sinks, such as forced air heat sinks, are preferred in applications requiring 26 W/in.2 or less of heat dissipating capability because of their cost effectiveness and reliability. However, the actual cost effectiveness and operating performance of convection heat sinks depends on a potentially complex set of variables, including heat sink material selection and base/fin fabrication details.
In a common convection-based heat sink configuration, the heat sink comprises a base and one or more outwardly extending fins. For example, one side of a flat base can be used to attach the device to be cooled, such as a power transistor device, with the other side of the base having some number of outwardly projecting fins to increase the surface area of the heat sink.
Assuming good thermal bonding between the base and the device to be cooled, good thermal performance of the overall heat sink depends on having low thermal impedances between the base and its fins. That is, the fins' ability to dissipate heat flowing into the base from the device to be cooled depends on maintaining good thermal conductance through the base into each fin. Different base/fin fabrication techniques offer a mix of compromises and advantages regarding thermal performance, overall cost, complexity, and manufacturability.
For example, extrusion allows formation of a heat sink base with integral fins. Extruding the heat sink as a single piece offers excellent thermal conduction from the base into each fin, because each fin represents a continuous extension of the base material. However, the overall thermal performance of one-piece extruded heat sinks of this type is limited by practical manufacturing considerations. Namely, the practical fin height is limited by extrusion limitations, such as the fin height-to-thickness aspect ratio (e.g., 20:1 maximum). Typical fin height-to-thickness aspect ratios of up to 6:1 and a minimum fin thickness of 1.3 mm are attainable with a standard extrusion process. An aspect ratio of 10:1 and a fin thickness of 0.8″ can be achieved with special die design features. However, increasing the aspect ratio compromises extrusion tolerances.
Practical manufacturing considerations of this sort generally limit the thermal performance of one-piece extruded heat sinks to heat dissipation to less than 12 W/in.2 maximum. Fabricating the base and fins as separate elements eliminates the extrusion-related fin height limitations inherent in the one-piece design. However, the two-piece approach presents the two-fold challenge of establishing good thermal conduction at the base-fin connections while maintaining requisite base-fin mechanical strength.
Employing a “conical press fit” method for securing the fins to the base offers relatively good thermal performance (in the range of 26 W/in.2), while simultaneously offering good mechanical performance. However, because the process relies on press fitting each fin into a “coned” aperture in the base, the fin insertion depth needed to securely seat each fin varies with fin and aperture dimension tolerances. Variations in the insertion depth translate into variations in seated fin height and generally, which requires the additional manufacturing step of cutting the fins down to a uniform, finished height. Overall, the precision and finishing required in this process makes it relatively expensive in comparison to other methods.
Bonding fins to the base rather than press fitting effectively eliminates the height variation problem, offers decent thermal performance (in the range of 24 W/in.2), and can reduce costs. Commonly, bonded heat sinks use thermally conductive aluminum-filled epoxy to bond planar fins onto a grooved extrusion base plate. This process allows for a much greater fin height-to-thickness aspect ratio of 20:1 to 40:1, which increases the cooling capacity without increasing volume requirements. However, thermal performance is heavily dependent on the bonding material, which itself can be expensive, and trade-offs exist between the thermal and mechanical performance of the bonding material.
Another two-piece alternative overcomes some of the limitations of both conical-press-fitting and epoxy bonding. This alternative uses “two-sided cold welding” to connect each fin to the base. One end of each fin includes ridges that are oversized relative to the insertion apertures of the base, so that inserting the fin into the aperture produces cold-welding on both sides of the inserted end of the fin. While this process produces good mechanical bonding, the irregular (non-conformal) contact between fin and base caused by the cold-welding ridges limits the thermal performance (in the range of 25-26 W/in.2).
Other processes exist, such as the use of a continuous sheet or plate “folded” into a series of fins, where the folded fin assembly is then attached to a base, or is otherwise bonded to the device to be cooled. However, folded fin assemblies generally suffer significant pressure drop in forced air convection applications, and do not offer good natural convection, leading to potentially serious problems in the event of fan failure.
On that point, airflow performance generally stands as a further complicating aspect of achieving good thermal performance in convection-based heat sinks, folded or otherwise. As a general proposition in forced air heat sinks, airflow must be ducted into the heat sink with sufficient velocity and volume to achieve good thermal performance. The ability of a fan to move high volumes of air is a function of pressure drop. As pressure increases, airflow decreases. Thus, reducing pressure drop in the heat sink assembly represents one approach to gaining significantly improved thermal performance. Of course, other factors, such as flow turbulence come into play as well.
With these complexities in mind, it will be appreciated that building a high performance heat sink represents a rather complicated design challenge. To meet that challenge, heat sink designers and fabricators must artfully balance cost and manufacturability against thermal and mechanical performance requirements.